The invention relates to vibrating beam force-frequency transducers which can constitute force-measuring devices of very small mass and volume.
More precisely, the invention relates to transducers of the type comprising a flat elongate blade for interposition between two elements for applying a longitudinal force to the blade, a middle portion of said blade constituting two lateral beams which are separated by a gap and which are interconnected by terminal portions of the blade, and which carry means for setting the beams into vibration in major of the large faces of the blade and means for measuring the frequency of vibration.
Such a transducer makes it possible to measure a traction or compression force applied to the beams. A particularly important, although non-exclusive, application lies in non-servo-controlled pendulum accelerometers having a seismic mass connected to a base via a hinge enabling it to act as a pendulum in a sense direction orthogonal to the hinge. The elements for applying the longitudinal force are then constituted by the base and the seismic mass.
Numerous transducers of the above type are already known. In such transducers, the terminal portions are designed to be secured to the force-applying elements. An embodiment is to be found in documents U.S. Pat. No. 3,238,789 and FR-A-2 454 614 (U.S. Department of Energy). FIG. 1 of the present application, which is not to scale for reasons of clarity, shows the mechanical portion of such a transducer 10 mounted between a base 12 and a pendulum seismic mass 14 connected to the base by a hinge 16. The transducer is constituted by an elongate quartz blade whose terminal portions provide respective connections (that can be considered as being fixed-end connections) both to the base and to the pendulum seismic mass. In the middle portion of the blade, a slot 18 separates two lateral beams 24a and 24b. In general, the blade is made of a piezoelectric material, e.g. quartz, and the means for vibrating the beams in their plane are constituted by electrodes (not shown) connected to a power supply and measurement circuit.
Such known transducers suffer from various drawbacks, associated in particular with the fact that the two beams cannot be made absolutely identical to each other and consequently have slightly different intrinsic resonant frequencies. For miniature transducers made by photolithography, having a resonant frequency of about 35 kHz, the resonant frequency f0a of one of the beams may differ from the resonant frequency f0b of the other beam by several tens of Hz. Because of the coupling between the beams, they vibrate at a single frequency f0 intermediate between f0a and f0b. This frequency is slightly different from the resonant frequency of each of the beams, thereby significantly lowering the Q-factor of the beams.
Another effect of this asymmetry is that interfering forces prone to be applied orthogonally to the sensitive axis x, and in particular in the direction z parallel to the plane of the blade, influence the beams differently.
If such an interfering force Fz is exerted along the axis z at the roots of the beams, then the force gives rise to traction in one of the beams and to compression in the other, and the smaller the distance between the beams the greater the forces. Under the effect of such interfering forces, the resonant frequency f0b increases under the effect of the traction while the resonant frequency f0a decreases under the effect of the compression. The force F.sub.2, depending upon its direction, can thus either increase the initial unbalance or tend to cancel it. In the first case, the Q-factor decreases, while in the second case it increases and tends towards its maximum. In either case the frequency slips so that there is a direct error in the output. These phenomena are square-law phenomena: if the force doubles then the frequency slip quadruples.
The appearance of such interfering forces in the fixing zones is inevitable. In the configuration shown in FIG. 1, any asymmetry between the ends of the hinge 16 will give rise, for example, to asymmetry in stiffness. During variations in temperature, a varying moment appears in the terminal portions about the axis y, thereby giving rise to interfering variation in the frequency f0 of the transducer.
Proposals have already been made to reduce these defects by balancing the beams by depositing gold and then vaporizing it in part with laser shots. That solution is not totally satisfactory. The load of gold on the beams reduces Q-factor. The volatilization of gold layers disturbs the underlying surface layers of quartz and also reduce Q-factor. Balancing the resonant frequencies at a given temperature does not eliminate drawbacks due to differences in the lengths of the beams.